A freed US hostage has reportedly told doctors he was tortured, beaten and held in a cage after trying to escape his Taliban captors.

Sergeant Bowe Bergdahl was released last weekend following five years of captivity in Afghanistan after the US agreed a controversial prisoner swap.

Now an unnamed senior US official who has been briefed on Sgt Bergdahl's progress at the Landstuhl Regional Medical Center in Germany says the 28-year-old apparently told staff he was put in a cage in total darkness for weeks as punishment for a foiled escape attempt.

Military doctors quoted in The New York Times say that while Sgt Bergdahl is physically able to travel he is not yet emotionally prepared to be reunited with his family.

He has received a letter from his sister but has not yet replied or spoken to relatives.

"Physically, he could be put on a plane to the US tomorrow, but there are still a couple of mental criteria to address: the family unification piece and the media exposure piece," the newspaper quoted an American official as saying.

Meanwhile, the FBI is investigating four emailed death threats sent to Sgt Bergdahl's father, Bob.

Sgt Bergdahl was returned to the US military in exchange for five Taliban members from Guantanamo Bay.

Opinion is split in the US over whether trading prisoners with the Taliban is appropriate.

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A Reuters/Ipsos poll found 44% disagreed with the statement that trading Taliban prisoners for Sgt Bergdahl was "the right thing to do". Twenty-nine percent said they thought the prisoner swap was the right thing to do and 27% said they were not sure.